In the presently existing public switched telephone network (PSTN), a network of interconnected switches couple communication stations, such as telephones, fax machines and computers, to one another. Each switch is uniquely identified by a 3-digit area code and a 3-digit exchange. The switches are coupled to the station devices through 4-digit extensions. Thus, when a calling party initiates a call to 303-555-1111, the switch assigned to 303-555 provides a connection to the station device (or devices) connected to extension 1111.
FIGS. 1a and 1b illustrate an exemplary portion of the PSTN, showing the common channel interoffice signalling system. The PSTN 10 includes a plurality of switches 12, each associated with a unique area code and exchange. For example, switch 12a is assigned the area code "801" and the exchange "555" Switch 12b is assigned the area code "801" and the exchange "556". Switch 12c is located in a different area code, "802", and has an exchange of "445". Each switch is coupled to communication devices 14. (Although shown as telephones, the station devices may be any device which is properly connected to the PSTN, such as computers, terminals or fax machines). Trunks 16 couple various switches 12. In order to establish communications between station devices, control and addressing signals are used. In the common channel interoffice signalling system, the control signals are separated from the voice/data signals. The control signals are sent over a separate circuit which controls the switching lines independently from the voice/data signals. These control signals can be passed directly between switches or transferred by a signal transfer point (STP).
As shown in FIG. 1a, "John Smith" has an assigned number of 801-555-1111 and "Rick Case" has an assigned number of 801-556-2221. If John Smith calls Rick Case, then John Smith dials "556-2221" (in this example, it is assumed that this connection is a local call and an area code need not be dialed). When the numbers are received, switch 12a communicates through STP 18a to establish a link with switch 12b over trunk 16a. Switch 12b establishes the link to the station at extension "2221".
While the PSTN is efficient at establishing connections between stations, one recognized inefficiency is the inability for telephone numbers to move along with their subscriber. In FIG. 1b, Rick Case has moved out of the area serviced by switch 12b and John Smith has moved into the area serviced by switch 12b and out of the area serviced by switch 12a. As a consequence, John Smith has been assigned a new exchange and extension, shown in FIG. 1b as "556-2224" Because Rick Case has moved out of the area serviced by switch 12b, his former number of "556-2221" is taken out of service for a predetermined time. During this time the number "801-556-2221" cannot be assigned to a new subscriber, since it is likely that calls to the old subscriber will continue to occur. Similarly, John Smith's previous number of "555-1111" is taken out of service. Consequently, when John Smith moves into the area serviced by "801-556", he is assigned a new number of "556-2224". Assuming Joe Smits moves into the house previously occupied by John Smith, switch 12a will reassign the physical connection between switch 12a and the house to a new extension, shown in FIG. 1b as "1114". Otherwise, Joe Smits would receive all calls for John Smith from people who do not know the number had changed.
The lack of portability of telephone numbers can be frustrating, and in many commercial scenarios, costly.
One solution to the problem of non-portable numbers is permanent "call forwarding", in which a call placed to the old number will result in a second connection (or more, if necessary) to the currently assigned number of the subscriber. Call forwarding, however, has several drawbacks. First, for each call forwarding operation to obtain the subscriber's current number, an otherwise usable number (the old number) is taken out of circulation. Second, maintaining strings of call forwarding numbers is burdensome on the PSTN.
A second approach is described in U.S. Pat. No. 4,754,479 to Bicknell et al. This approach uses routing numbers which differ in format from common format numbers (i.e., those using the 10-digit format of NNA-NNX-XXXX, where NNA is the area code, NNX is the exchange and XXXX is the extension), such that the number types identify a number which is not assigned to a physical switch. These approaches require either the use of a different parameter for call routing (e.g., a service provider identifier) or the use of a unique number format. The use of a different parameter for call routing can result in expensive call routing inefficiencies which would be incurred due to the inability of the number to identify the actual terminating switch for the call; in other words, more precise routing information is required beyond just identifying the terminating access provider. In the case of providing a unique format number, such as a 6-digit number, the proposed routing number may be able to identify the actual switch; however, there are substantial impacts to all involved transport networks as well as to the terminating switch. Thus, the signalling, translation and routing of a new routing number requires new tandem switching functionality which does not exist currently in at least most, if not all, of the switch-types available from the switch vendor community. Also, the use of a routing number that only identifies the terminating switch requires the use of a parameter to enable the terminating switch to perform the final translation to identify the line/trunk appearance for the call.
Therefore, a need has arisen for a method and apparatus for routing portable telephone numbers which are compatible with existing switching facilities.